feat (prometheus): membership metric

Add a gauge metric `membership` to the `_prometheus` endpoint. The metric
has labels:

- `nodes=all_nodes`
- `nodes=cluster_nodes`

matching the fields in the `_membership` endpoint (I think consistency
here is more useful than renaming the labels to e.g. expected/actual).

1 files changed, 13 insertions, 1 deletions

diff --git a/src/couch_prometheus/src/couch_prometheus_server.erl b/src/couch_prometheus/src/couch_prometheus_server.erl
index 0fd169404..05cd26265 100644
--- a/src/couch_prometheus/src/couch_prometheus_server.erl
+++ b/src/couch_prometheus/src/couch_prometheus_server.erl
@@ -113,7 +113,8 @@ get_system_stats() ->
         get_run_queue_stats(),
         get_vm_stats(),
         get_ets_stats(),
-        get_internal_replication_jobs_stat()
+        get_internal_replication_jobs_stat(),
+        get_membership_stat()
     ]).
 
 get_uptime_stat() ->
@@ -127,6 +128,17 @@ get_internal_replication_jobs_stat() ->
         mem3_sync:get_backlog()
     ).
 
+get_membership_stat() ->
+    % expected nodes
+    ClusterNodes = mem3:nodes(),
+    % connected nodes
+    AllNodes = nodes([this, visible]),
+    Labels = [
+        {[{nodes, "cluster_nodes"}], length(ClusterNodes)},
+        {[{nodes, "all_nodes"}], length(AllNodes)}
+    ],
+    to_prom(membership, gauge, "count of nodes in the cluster", Labels).
+
 get_vm_stats() ->
     MemLabels = lists:map(
         fun({Type, Value}) ->